1. Field of the Invention
The present invention relates to an optical element including an anti-reflection film, and an optical system and an optical apparatus using the same.
2. Description of the Related Art
Conventionally, an anti-reflection film for preventing the loss of the light intensity of incident light has been coated on the surface of the optical element employed in the imaging optical system provided in a photographing lens for use in an optical apparatus such as a video camera, a photographic camera, a television camera, or the like. For example, a dielectric multilayer film (typically called “multi-coating”) is widely employed as an anti-reflection film for an optical element for visible light. The dielectric multilayer film is formed of thin films, each having a different refractive index and an appropriate thickness, stacked together, whereby the amplitudes and the phases of reflected waves generated on the surfaces and the interfaces of the films are adjusted and made to interfere with each other so as to reduce reflected light. The anti-reflection film formed of the dielectric multilayer film exhibits excellent anti-reflection performance with respect to a light beam having a specific wavelength or at a specific incident angle. However, since the interference condition is not met for other light beams, it is difficult for the anti-reflection film to realize high anti-reflection performance across a wide wavelength band or a large incident angle range.
On the other hand, in a recent digital camera, an image sensor such as a CCD or a CMOS having a reflectance higher than that of a silver salt film has been used. Thus, a specific ghost called a “digital ghost” caused by the light, which has been reflected from the sensor surface of the image sensor, reaching the sensor surface again after being reflected from a lens (optical element) surface may readily occur. Also, as a lens employed in the digital camera, an abnormal dispersion glass, an aspherical lens, a lens having a large curvature, or the like may be often used in order to simultaneously achieve high quality images or high specification (zoom magnification or brightness) and portability (size reduction or weight reduction). In particular, in a lens having a large curvature, a light beam is incident at a large angle to the peripheral part of the lens. Therefore, the conventional anti-reflection film formed of the dielectric multilayer film as described above cannot prevent the reflection of light, resulting in the occurrence of unnecessary light, such as flare or ghost, which may adversely affect the resultant quality of photographed images.
Therefore, there has been a demand for an anti-reflection film which is excellent in wavelength band characteristics and incident angle characteristics, and Japanese Patent No. 4433390 discloses an anti-reflection film in which a magnesium fluoride layer is formed by a sol-gel method on a three-layered dielectric thin film formed by a vacuum deposition method and an optical element having the anti-reflection film.
Here, in the vacuum deposition method disclosed in Japanese Patent No. 4433390, an anti-reflection film is formed by a deposition material that is ejected from a deposition source and is deposited on a lens. In this case, assume that the film thickness at a position perpendicular to the deposition source is defined as 1, the film thickness at a position inclined at an angle of θ is theoretically cos θ. In other words, when a film is deposited on a lens having a large curvature using a vacuum deposition method, the thickness at the peripheral part of the lens is thinner compared with that at the central part thereof. Thus, if an anti-reflection film is formed by applying the film forming method disclosed in Japanese Patent No. 4433390 to a lens having a large curvature, the thickness at the peripheral part of each of the first layer to the third layer disclosed in Japanese Patent No. 4433390 becomes thin, and thus, the interference condition is not met, resulting in a high probability of adversely affecting anti-reflection performance.
In contrast, for example, it is also contemplated that the thickness at each of the central part and the peripheral part of a lens may be made uniform by employing a method in which a shielding having an appropriate opening is provided between a deposition source and a lens, and a deposition material is deposited through the shielding onto the lens while rotating it. However, if such a method is employed, the deposition material to be deposited onto the lens may adhere to the shielding, resulting in a reduction in the film-forming efficiency. In addition, the number of lenses housed within a deposition apparatus is reduced so as to ensure a space for installing a rotation mechanism, resulting in a reduction in productivity.